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How and why the Sun shines and stars twinkle are questions that have always intrigued the human mind. For a long time it was believed that the Sun shone by actually burning its own material through combustion - just like a fire in the fireplace. However, by the mid 19th century geologists were suggesting that a few hundred thousand years were needed for the important features on the Earth´s surface to form and it was realized that the Sun would have needed inconceivable amounts of fuel to have kept burning that long.
Lord Kelvin and Helmholtz elaborated a theory that the Sun could generate energy by its own gravitational collapse. But this would only generate energy for 20 million years. In fact, Lord Kelvin argued against Darwin´s theory of evolution because he believed that there was not enough time for it to have taken place. Today the we know that our sun is about five billion years old.
With the arrival of the new understanding of the atomic and nuclear nature of matter at the beginning of the 20th century, the scene was set for a real understanding of fusion as the power source of the stars. The first clues to how the stars function were revealed in Einstein´s deceptively simple equation E = mc2 derived in 1905 as a consequence of his special theory of relativity. This famous equation predicted that a tiny amount of mass could, in principle, be converted into a tremendous amount of energy.